1. Field of the Invention
The present invention relates to an underground-environment simulator which simulates underground environment spaces used for radioactive waste disposal or the like. More particularly, the present invention relates to an underground-environment simulator having a box whose atmosphere is controllable, and being substantially free from stagnation of the gas flow inside the box.
2. Description of the Related Art
Recently, various investigations are being conducted concerning disposal of highly radioactive wastes derived from the nuclear fuel cycle. In such investigations, there is an increasing demand for actualizing, in a simulating manner, an underground environment (with a low oxygen concentration and an optional low carbon dioxide gas concentration) at a depth of a few hundred meters or more where such wastes are disposed, and performing experiments in such an environment.
Such experiments are carried out using a hermetic box, the atmosphere inside which can be isolated from the surroundings. When a radioactive material is treated in the box, the inside of the box is always maintained at a negative pressure in order to prevent external leakage of the radioactive material. For control of the atmosphere inside the box, an inlet pipe and an exhaust pipe are provided on an outer wall of the box, a gas-controlling unit is connected to such inlet pipe and exhaust pipe, and thus an underground-environment simulator is constructed.
Hitherto, in such an experimental facility, a vacuum state is achieved by exhausting the gas inside the hermetic box 51, or most parts of the oxygen and carbon dioxide gas are removed by feeding an inert gas such as nitrogen from an inert gas feeding unit 52 after the inside of the box is purged with nitrogen at ordinary pressure, as shown in FIG. 3. Subsequently, while the inert gas fed into the hermetic box 51 is circulated together with the remaining oxygen and carbon dioxide gas, the remaining oxygen is removed through a deoxygenating unit 53 provided in the circulation path. Further, moisture and the remaining carbon dioxide gas is removed through a water adsorbing unit 54 and a carbon dioxide gas adsorbing unit 55 which are connected in parallel. As a result, an underground environment at an extremely deep place with extremely low oxygen and carbon dioxide gas concentrations is achieved (cf. Japanese Unexamined Patent Publication No. 1-207748).
Actual underground environments, however, differ in carbon dioxide gas concentration according to depth and geological features, though the oxygen concentrations are similarly low substantially without being affected by temperature. When actual underground environments which differ in gas concentration as above should be more closely simulated, the above-described continuous removal of oxygen and carbon dioxide gas by circulation is insufficient. In particular, carbon dioxide gas concentration is a factor influencing experimental results of radioactive wastes greatly even with an only slight variations thereof, and highly accurate experiments require simulation of underground environments with controlled or varied carbon dioxide gas concentration.
Additionally, also in the hermetic box, the atmosphere is needed to be controlled so as to uniformly and stably retain a predetermined gas composition. However, merely adjusting the connecting positions of the gas inlet pipe and the exhaust pipe relative to the outer wall of the hermetic box could rarely achieve uniform distributions of the gases circulating in the box through these pipes. For example, a specific gas can be unevenly distributed at the corner portions of the box.
More particularly, such a hermetic box is provided with openings for gloves needed for experimental operations, a monitoring window, connecters for connecting external measuring equipment, and others, and these portions have sealing regions for preventing external air from entering. Although such sealing regions comprise sealing materials such as natural rubber, neobutylene, teflon, hydrochlorinated rubber and butyl rubber, oxygen in the air can permeate such sealing materials and enter into the inside of the box. In addition, since the inside of the box used for treatment of radioactive materials is set at a negative pressure, external air can leak into the box through the sealing regions when deterioration occurs thereat. As a result of leakage and entering of oxygen or the like by such permeation, gas concentrations readily become uneven in positions close to the internal walls of the box. Further, when drifting or whirling occurs in the gas flow inside the box, the internal atmosphere can rarely be controlled to maintain a predetermined composition during operation, and a long time period is needed for the start-up of operation to achieve a predetermined internal atmosphere.